The respiratory mucosa employs the innate and adaptive immune system to protect normal respiratory function from invading organisms. However, when there is a defect in one of these defenses the host becomes vulnerable to Aspergillus fumigatus (Af). This ubiquitous fungus enters the airways as a spore, or resting conidium but is generally cleared by intact respiratory defenses. However, when individuals are immunocompromised, Af conidia are more likely to germinate and form filamentous structures called hyphae. As this morphotype, Af can become invasive particularly in persons with low neutrophil counts. An estimated 200,000 people are diagnosed with invasive aspergillosis annually worldwide, and up to 90% die from the infection. Af can also colonize the airways of those who suffer from asthma or cystic fibrosis causing allergic bronchopulmonary aspergillosis (ABPA), which affects over 4 million people annually worldwide. Studies proposed here seek to further understand the intact innate immune response to Af conidia, particularly the involvement of interleukin-23 (IL-23) and interleukin-17 (IL-17). A better understanding of this response may uncover nuances associated with the host defects that predispose to infection with Af, as well as potentially inform rational vaccine design and immunotherapies against Af. Af conidia elicit IL-23 and IL-17 production from the host airways within the first 24 hours of infection. These cytokines are known to be important for the adaptive TH17 response. However their role in innate immunity against Af is largely unknown. IL-23 has been shown to augment IL-17 production, and in turn IL-17 elicits neutrophil recruitment. The relationship between IL-23 and IL-17 is referred to as the IL-23/IL-17 axis and this proposal aims to systematically characterize each portion of this axis in the innate response against Af conidia, and test whether this response is required for protection against this mycosis. In order to characterize the temporal production pattern of IL-23, we will measure levels of this cytokine at regular intervals in the fist 72 hours of infection by ELISA. From a preliminary screen, we have uncovered candidate cell types that may be involved in the production of IL-23. We aim to confirm these sources by in vivo and ex vivo intracellular cytokine staining. To test whether IL-23 production is protective against mortality in Af infection, the survival rates of wild-type (WT) mice will be compared to a functional IL-23 knock-out strain (IL-23p19-/-). Finally, we propose to create mixed bone marrow chimeras to test whether any specific cellular source of IL-23 is required for protection against Af (Specific Aim 1). The temporal pattern of production for IL-17 and its source will also be characterized in the first 72 hours of infection with Af using methods described above. To test whether IL-23 augments IL-17 production innately in response to Af, IL-17 levels will be assessed in IL-23p19-/- mice and WT mice. In addition, we have evidence that IL-23 and IL-17A are co-produced by one innate cell type in response Af, we propose to test and dissect any potential autocrine mechanisms in this cell type. Finally, the role of IL-17 in protection against f infection will also be tested by monitoring the survival of IL-17RA-/- mice and WT mice (Specific Aim 2). Because many at risk for IA are transplant patients who are iatrogenically immunosuppressed, knowledge of the factors leading to protection against aspergillosis could also inform development of targeted immunosuppressive agents that keep defenses against opportunistic infections intact.